Stable suspensions of chargeable pigment particles are desirable in certain applications wherein the formation of images (permanent or latent) requires charging or discharging the pigment particles. Such applications include liquid toners, ink jet inks using solvents and electrostatic paints.
The preparation of such suspensions has been documented in literature. To stabilize hydrophilic pigments in non-polar solvents, steric barriers are introduced to provide stabilization. Common strategies to do so include the use of ionic surfactants or polymeric dispersants which adsorb on the pigment (e.g., U.S. Pat. Nos. 5,536,615 and 5,482,809); dendrimers (e.g., U.S. Pat. No. 5,558,968) or non-polymeric resins (e.g., U.S. Pat. No. 5,521,046) to modify the pigment surface. Though polymeric dyes (U.S. Pat. Nos. 5,521,271 and 5,563,024) have been disclosed as alternatives to pigments for use in such applications, their inferior lightfastness and weatherability, coupled with their relatively high production costs, have restricted their usage.
Stable suspensions of chargeable pigment particles in non-polar solvents is difficult to prepare, mainly for two reasons. First, many chargeable pigment particles contain surface polar functional groups like carboxylic acid. In addition to rendering the pigment chargeable, these polar functional groups also make the pigment hydrophilic. In non-polar systems containing non-polar solvents, the hydrophilicity of the pigment particles causes them to be attracted to each other rather than to the non-polar solvent, resulting in flocculation and destabilization of the pigment suspension.
Second, in contrast to the possibility of applying both steric and electrostatic interactions to combat flocculation in polar systems, mainly steric interaction can be applied in non-polar systems. In most cases, surfactants are used to provide steric stabilization in suspending such particles. The polar ends of the surfactants adsorb onto the hydrophilic pigment surface, and the hydrophobic portion is solvated by the non-polar solvent. This mode of weak attachment leads to flocculation and degradation of the non-polar suspension. A disadvantage of using physically adsorbed surfactants or dispersants in suspensions is that under external stress, like a high electric field, some of these suspensions or formulations become unstable. This is due to the dissociation or desorption of a surfactant or resin from the pigment particle surface.
The use of dendrimers that are covalently bonded to pigment surfaces partially addresses the problem of desorption, but the process of assembling such a complex molecular architecture on the pigment surface translates into a lengthy preparation and high production cost.
A need therefore exists for a method of making a non-polar suspension of chargeable pigment particles that does not suffer from problems such as desorption, and is available as a lower cost alternative to the use of dendrimers and the like.